1. Field of the Invention
The present invention relates to a solid electrolytic capacitor including an anode body having a dielectric coating, a solid electrolyte layer and a cathode extraction layer successively formed thereon.
2. Description of the Related Art
At present, a rolled body of roughened aluminum foil, a monolayer or multilayer body of a roughened aluminum sheet, or a porous sintered body of tantalum powder, for example, is frequently used as an anode body of a solid electrolytic capacitor.
Niobium is also receiving attention as a material of the anode body. Niobium is a metal belonging to the same 5A group of the periodic system as tantalum and having a physical property similar to tantalum. Because niobium has many advantages as compared to tantalum such as a smaller specific gravity, larger reserves and a lower price per kg, attempts have been made to utilize niobium as an anode material of an electrolytic capacitor.
A conventional solid electrolytic capacitor has a structure such that, as shown in FIG. 1, on a surface of an anode body 1 formed with a valve action metal such as tantalum, aluminum or niobium, a dielectric coating 2 formed by oxidization of the anode body 1, a solid electrolyte layer 3 formed with a conductive inorganic material such as manganese dioxide or a conductive organic material such as a TCNQ (7,7,8,8-tetracyanoquinodimethane, which is the same in the following) complex salt or a conductive polymer, and a cathode extraction layer 4 formed with conductive carbon, silver or the like are successively formed to construct a capacitor element 10, an anode terminal 61 is connected by resistance welding or the like to an anode lead member 11 inserted into the anode body from one end surface thereof, a cathode terminal 62 is connected to cathode extraction layer 4 using a conductive adhesive 5, and capacitor element 10 is covered and sealed with an exterior resin 7 such as an epoxy resin formed by injection molding or the like (see, for example, Japanese Patent Laying-Open No. 10-064761 (in particular, page 2 and FIG. 1 )).
In the solid electrolytic capacitor described above, formation of the exterior resin using a highly durable hard epoxy resin and application of a heat load of a high temperature for lead-free solder degrade insulation of the dielectric coating and increase an LC (Leaked Current, which is the same in the following) value.
In particular, it is repeatedly reported in conferences that such increase in the LC due to the heat load or the like is much larger in a solid electrolytic capacitor using an anode body including niobium as a main component as compared with that using tantalum in the anode body. In the solid electrolytic capacitor using the anode body including niobium as a main component, the dielectric coating is nearly short-circuited due to the heat load or the like, and the LC value is increased so much that it cannot be restored by aging (gradually decreasing the LC with a voltage load, which is the same in the following). A main cause thereof is reported to be diffusion of a portion of oxygen atoms of the dielectric coating (Nb2O5) among niobium (Nb) of a base side and resulting lack of oxygen in the dielectric when excessive heat is applied to the capacitor element, which is a phenomenon unique to niobium (for example, see the Electrochemical Society of Japan, “Capacitor Technology” Vol. 9, No. 1, 2002, p. 2).
A conceivable method for restoring the dielectric coating may be to supply lacked oxygen or to convert a portion of the solid electrolyte contacting a defective portion locally into a nonconductive form. It is difficult, however, to supply oxygen to the solid electrolyte not containing water, and localized conversion of the solid electrolyte, especially the conductive polymer, into a nonconductive form cannot be performed in a short time.